An early and essential event in mammalian embryonic brain development is neuronal polarization, in which distinct axonal and dendritic compartments are formed that inherently differ in their molecular composition. These differences underlie the unique morphology and function of these compartments, and are responsible for directed information flow in the brain. Aberrations in neuron polarization lead to developmental neuropathologies, intellectual disability, epilepsy, neuropsychiatric and autism spectrum disorders. Bipolar polarity establishment in neocortical and hippocampal CA1 Pyramidal neuron progenitors marks axon/apical dendrite polarity - the apical dendrite will develop from the leading process whereas the trailing process will become the axon. Specification of the axon has dominated studies on neuron polarization, yielding an understanding of the mechanisms underlying axonal identity, its specification and growth. Much effort has also been directed towards elucidation of the mechanisms that control later events in dendrite morphogenesis - growth, branching, and structural plasticity. However, the events leading to bipolar polarity and the subsequent development of the apical dendrite, remain elusive. We propose that distinctly higher cyclic GMP (cGMP) generated via localized assembly of a cGMP production machinery at the leading edge of developing pyramidal neurons, promotes bipolar polarity, leading process formation, and apical dendrite development. Using state of the art lifetime decay FLIM-FRET cGMP measurements in mouse developing pyramidal neurons in acute slice, combined with cutting edge genetic approaches, and localized optogenetic manipulations of cGMP production, this study is designed to determine the spatio-temporal regulation of cGMP during polarity establishment and apical dendrite development and to identify its mechanistic basis in developing pyramidal neurons in vivo. These studies will provide important advance in the understanding of the early molecular events that take place during axon/apical dendrite development in principal excitatory neurons in the rodent brain.